The key difference between allylic and benzylic halides is that allylic halides contain a halogen atom bonded to an allylic carbon atom whereas benzylic halides contain a halogen atom bonded to a benzylic carbon atom.
An allylic carbon atom is the carbon atom that is adjacent to a double bond in an organic compound while a benzylic carbon atom is the carbon atom that is adjacent to a benzene ring.
CONTENTS
1. Overview and Key Difference
2. What are Allylic Halides
3. What are Benzylic Halides
4. Side by Side Comparison – Allylic vs Benzylic Halides in Tabular Form
5. Summary
What are Allylic Halides?
Allylic halides are organic compounds having the chemical formula R=R’-R”X. In other words, allylic halides have one or more halogen atoms on allylic carbons. A common example is allylic chloride compound.
Figure 01: Structure of Allyl Chloride
Allyl chloride compound contains its chlorine atom bonded to the carbon atom that is adjacent to the double bond in the molecule. In other words, allyl chlorides are alkenes containing a chlorine atom. In this molecule, the chlorine atom is bound to the carbon atom that is nearest to the double bond of the alkene. Although carbon atoms having the double bond are sp2 hybridized, the carbon atom bearing the chlorine atom is sp3 hybridized.
Furthermore, this carbon atom bonds with the double-bonded carbon atom through a single bond. Therefore, the electron density around this carbon atom is lower than that of the carbon atoms in the double bond. If a molecule contains two double bonds, then the allylic carbon that bears the chlorine atom can act as a bridge for two double bonds.
What are Benzylic Halides?
Benzylic halides are organic compounds having one or more halogen atoms on benzylic carbons. In other words, the chemical structure of benzylic halides is C6H5C-X; here, one or more halide atoms are bonded to a carbon atom that is located adjacent to a benzene ring. Therefore, the benzyl carbon atom is the carbon atom that is directly attached to a benzene ring. We can abbreviate benzyl group as “Bn” and benzyl halide as “Bn-X” – X refers to a halide atom. The most common benzyl halide is benzyl chloride compound.
Figure 02: Structure of Benzyl Chloride
Benzyl chloride occurs as a colourless liquid that is highly reactive. It is important in the chemical synthesis processes. We can prepare benzyl chloride industrially via the gas phase photochemical reaction of toluene and chlorine gas. This liquid has a pungent odour.
What is the Difference Between Allylic and Benzylic Halides?
An allylic carbon atom is the carbon atom that is adjacent to a double bond in an organic compound while a benzylic carbon atom is the carbon atom that is adjacent to a benzene ring. The key difference between allylic and benzylic halides is that allylic halides contain a halogen atom bonded to an allylic carbon atom whereas benzylic halides contain a halogen atom bonded to a benzylic carbon atom.
The below infographic tabulates more differences between allylic and benzylic halides.
Summary – Allylic vs Benzylic Halides
An allylic carbon atom is the carbon atom that is adjacent to a double bond in an organic compound while a benzylic carbon atom is the carbon atom that is adjacent to a benzene ring. The key difference between allylic and benzylic halides is that allylic halides contain a halogen atom bonded to an allylic carbon atom whereas benzylic halides contain a halogen atom bonded to a benzylic carbon atom.
Reference:
1. “Allylic Halide.” Chemistry LibreTexts, Libretexts, 24 Aug. 2020, Available here.
2. “Benzylic Halide.” OChemPal, Available here.
3. “Benzylic Halide.” Chemistry LibreTexts, Libretexts, 24 Aug. 2020, Available here.
Image Courtesy:
1. “Allyl chloride” By Cwbm (commons) – standard textbook (Public Domain) via Commons Wikimedia
2. “Benzyl chloride” By Edgar181 – Own work (Public Domain) via Commons Wikimedia